(a) Technical Field
The present invention relates to a method for using an amidium-based ionic liquid obtained by reacting amide and an organic acid as a carbon dioxide absorbent. More particularly, the present invention relates to a carbon dioxide absorbent, which has excellent carbon dioxide absorption capability, preferably comprising an amidium-based ionic liquid, which is suitably easy to synthesize and has a low manufacturing cost.
(b) Background Art
Methods of separating CO2 from exhaust gases released from large boilers, chemical plants, power plants as well as a natural gas include absorption, adsorption, membrane separation, and cryogenics. In certain cases, when the CO2 concentration being released is low, the method of absorption is preferred. The absorption method is commonly used in industry because it enables the selective separation of certain gases which are well absorbed to an absorbent. However, the absorbent that is used is partially inactivated during the separation and is thus required to be replaced periodically. Preferably, the absorption method employs a liquid absorbent and can be easily replaced, and therefore this method is used in the purification of a large amount of exhaust gases or in gas separation
Preferred examples of the carbon dioxide absorbent include aqueous amine-based solutions such as monoethanolamine (MEA), N-methyldiethanolamine (NDEA), diethanolamine (DEA). This is because, when the amine absorbent, which has weak alkalinity, is bound to CO2, which is an acidic gas, and is heat-treated, CO2 can be released and collected while the absorbent is recycled. However, when using this technology, the impurities such as SOx and NOx contained in the absorbed gas tend to decompose amines; heating the absorbent bound to CO2 for breaking the chemical bond between CO2 and the absorbent irreversibly decomposes amines and subsequently deteriorates the capability of the absorbents; the fresh absorbents need to be replenished; amines or their decomposed products cause corrosion of absorption apparatus; high vapor pressure of amines causes the contamination of the CO2 gas released.
To address the above-described properties of the above-mentioned aqueous amine-based absorbents, there have been reports on methods of physically absorbing CO2 by using an organic solvent such as Selexol, IFPexol, NFM, etc.
An important feature of the organic solvent as an absorbent is that CO2 absorption is proceeded via physical interaction between the solvent and CO2, not by the chemical bond as in the case of the aqueous amine-based absorbents, thus requiring a relatively low energy in CO2 recovery and solvent recycling.
In the case of using an amine-based absorbent, CO2 recovery and solvent recycling processes require stripping at high temperature, which is an energy intensive process. However, in the case of physical absorption CO2 can be recovered by means of changing pressure without increasing temperature.
Physical absorption has a low CO2 absorption capability. An organic solvent in general has much lower CO2 absorption capability than aqueous amine-based solution, and thus the circulation rate of an absorbent is high thereby requiring larger equipment.
Physical absorption has a high circulation rate. The physical absorption process by an organic solvent generally requires a circulation rate that is twice as high as that of an aqueous amine-based solution, thus requiring high cost for capital and operation & maintenance.
Physical absorption has a high loss of a solvent. The solvents used in physical absorption have a high vapor pressure and thus can be readily lost during the absorption and regeneration processes. The loss can be suitably minimized by a process of cooling or washing, but to do so requires the installation of additional equipment.
Therefore, there has been an urgent need for the development of a novel absorbent that can replace both amine-based absorbents and organic solvent absorbents, and can resolve the above-mentioned drawbacks of amine-based absorbents and organic solvent absorbents.
As a way to resolve the problems of the conventional absorbents, U.S. Pat. Nos. 6,849,774 and 6,623,659 and published U.S. Pat. Application No. 2008/0146849, incorporated by reference in their entireties herein, disclose attempts made to use ionic liquids, which have no volatility, but with high thermal stability, and maintain liquid state at low temperature of 100 C or below.
An ionic liquid is a salt compound consisting of organic cations and organic or inorganic anions where gas molecules such as CO, CO2, SO2, and N2O can be well dissolved.
The solubility of a gas to be absorbed into an ionic liquid varies depending on the degree of the mutual interaction between the gas and the ionic liquid. Therefore, the degree of solubility of a gas can be controlled to some extent by adjusting polarity, acidity, alkalinity, and nucleophilicity of an ionic liquid.
Examples of ionic liquids include, but are not limited to, organic cations containing nitrogen such as imidazolium, pyrazolium, triazolium, pyridinium, pyridazinium, pyrimidinium (a quarternary ammonium); halogens such as Cl−, Br−, and I−; and anions such as BF4−, PF6−, (CF3SO2)2N−, CF3SO3−, MeSO3−, NO3−, CF3CO2−, and CH3CO2−. In particular, anions containing F atom are known to have relatively high CO2 absorption power.
Ionic liquid absorbents have much lower CO2 absorption capability than the amine-based absorbent. Also, ionic liquid absorbents are detrimental to moisture content thus being readily decomposed by the water contained in a mixed gas, thereby gradually decreasing in absorption power. Ionic liquid absorbents are is not considered economical due to its high manufacturing cost.
The anions containing F atom such as tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), trifluorosulfonimide[(CF3SO2)2N−] have high solubility of the acidic gases including CO2 and CS2. However, the manufacturing process of the ionic liquid is rather complex requiring at least 2 steps and also its manufacturing cost is very high thus not suitable for industrial application.
The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.